It is to be expected that in the future battery systems or batteries on which very stringent demands in respect of reliability will be made will be used increasingly both in stationary applications and in vehicles such as hybrid and electric vehicles. The background for this consists in that a failure of the battery can result in a safety-relevant problem. In order to provide the powers desired for a respective application, in general a high number of battery cells are connected in series within a battery, as a result of which a high output voltage of the battery is produced, which, without any suitable measures, is present permanently at the corresponding supply lines of the apparatus supplied by the battery and can pose a risk to maintenance personnel or users. For this reason, contactors are generally provided in order to be able to electrically decouple the battery. In motor vehicles with an electric drive motor, contactors are usually installed both at the positive terminal and at the negative terminal of the battery, which contactors are rated for the high voltage of the battery and also need to be able to disconnect the battery reliably in the event of short-circuit currents of over 1000 A.
The switching-on and switching-off of contactors generally takes place via an electronic output stage or via a drive circuit, which supplies current to the drive coils of the contactors. The drive power is in this case not negligibly low. During a switch-on operation, however, much higher drive currents are required for reliable attraction of the contactors than for subsequently holding the contacts in the closed state. For this reason, it is conventional to divide the driving of the contactors into two modes, the attraction mode and the holding mode (also referred to as attraction phase and holding phase, respectively). An indication of the respective mode consists in the level of the drive current, which is higher during the attraction mode than during the holding mode. In this case, the terms attraction level and holding level are used. In this case, the attraction mode is only required for switching-on (closing) of the contactors and has a relatively short duration. For the majority of the use time, the contactors are operated in the power-saving holding mode. A drive circuit for driving contactors should therefore be capable of providing both operating modes.
DE 10 2010 041 018 A1 discloses an apparatus for driving a contactor which comprises a holding current unit, which is designed to output a holding current for the drive coil of a contactor at one of its output-side outputs. With the apparatus disclosed in DE 10 2010 041 018 A1, the driving of at least one contactor during the attraction phase and during the holding phase can be performed with different voltage levels of constant voltages in an advantageous manner.
However, the component parts used, for example, in the apparatus disclosed in DE 10 2010 041 018 A1, in particular the winding resistors of the drive coils of the contactors, have a temperature dependence and in each case a manufacturing-related scatter of their component part parameters. In addition, the holding voltage generated by the holding current unit is adjusted to a value which is fixed for the production time.
The component parts used therefore need to be configured in such a way that the required holding current can be provided by the holding current unit even in the case of the presence of extreme temperatures. Since, however, the conductivity of the component parts and as a result the current flow through said component parts, such as through the drive coil of the contactor, for example, fluctuates depending on the temperature, it is necessary to dimension the component parts to be larger than would be necessary for the actually desired current. For example, the component parts used in the holding circuit of the apparatus disclosed in DE 10 2010 041 018 A1 for this reason need to be dimensioned so as to be up to 66% larger, which significantly increases the required installation space and the costs for the component parts.